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tutorial: modernize the definition and use of Open.

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R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/4446053
This commit is contained in:
Rob Pike 2011-04-18 10:51:40 -07:00
parent beb64bbd6e
commit 23fc9c84bd
6 changed files with 111 additions and 51 deletions
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113
doc/go_tutorial.html
View File

@ -474,8 +474,8 @@ assigned to a variable.
<p>
<h2>An I/O Package</h2>
<p>
Next we'll look at a simple package for doing file I/O with the usual
sort of open/close/read/write interface. Here's the start of <code>file.go</code>:
Next we'll look at a simple package for doing file I/O with an
open/close/read/write interface. Here's the start of <code>file.go</code>:
<p>
<pre> <!-- progs/file.go /package/ /^}/ -->
05 package file
@ -554,10 +554,10 @@ We can use the factory to construct some familiar, exported variables of type <c
</pre>
<p>
The <code>newFile</code> function was not exported because it's internal. The proper,
exported factory to use is <code>Open</code>:
exported factory to use is <code>OpenFile</code> (we'll explain that name in a moment):
<p>
<pre> <!-- progs/file.go /func.Open/ /^}/ -->
30 func Open(name string, mode int, perm uint32) (file *File, err os.Error) {
<pre> <!-- progs/file.go /func.OpenFile/ /^}/ -->
30 func OpenFile(name string, mode int, perm uint32) (file *File, err os.Error) {
31 r, e := syscall.Open(name, mode, perm)
32 if e != 0 {
33 err = os.Errno(e)
@ -566,7 +566,7 @@ exported factory to use is <code>Open</code>:
36 }
</pre>
<p>
There are a number of new things in these few lines. First, <code>Open</code> returns
There are a number of new things in these few lines. First, <code>OpenFile</code> returns
multiple values, a <code>File</code> and an error (more about errors in a moment).
We declare the
multi-value return as a parenthesized list of declarations; syntactically
@ -585,6 +585,35 @@ consistent error handling throughout Go code. In <code>Open</code> we use a
conversion to translate Unix's integer <code>errno</code> value into the integer type
<code>os.Errno</code>, which implements <code>os.Error</code>.
<p>
Why <code>OpenFile</code> and not <code>Open</code>? To mimic Go's <code>os</code> package, which
our exercise is emulating. The <code>os</code> package takes the opportunity
to make the two commonest cases - open for read and create for
write - the simplest, just <code>Open</code> and <code>Create</code>. <code>OpenFile</code> is the
general case, analogous to the Unix system call <code>Open</code>. Here is
the implementation of our <code>Open</code> and <code>Create</code>; they're trivial
wrappers that eliminate common errors by capturing
the tricky standard arguments to open and, especially, to create a file:
<p>
<pre> <!-- progs/file.go /^const/ /^}/ -->
38 const (
39 O_RDONLY = syscall.O_RDONLY
40 O_RDWR = syscall.O_RDWR
41 O_CREATE = syscall.O_CREAT
42 O_TRUNC = syscall.O_TRUNC
43 )
<p>
45 func Open(name string) (file *File, err os.Error) {
46 return OpenFile(name, O_RDONLY, 0)
47 }
</pre>
<p>
<pre> <!-- progs/file.go /func.Create/ /^}/ -->
49 func Create(name string) (file *File, err os.Error) {
50 return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
51 }
</pre>
<p>
Back to our main story.
Now that we can build <code>Files</code>, we can write methods for them. To declare
a method of a type, we define a function to have an explicit receiver
of that type, placed
@ -592,43 +621,43 @@ in parentheses before the function name. Here are some methods for <code>*File</
each of which declares a receiver variable <code>file</code>.
<p>
<pre> <!-- progs/file.go /Close/ END -->
38 func (file *File) Close() os.Error {
39 if file == nil {
40 return os.EINVAL
41 }
42 e := syscall.Close(file.fd)
43 file.fd = -1 // so it can't be closed again
44 if e != 0 {
45 return os.Errno(e)
46 }
47 return nil
48 }
53 func (file *File) Close() os.Error {
54 if file == nil {
55 return os.EINVAL
56 }
57 e := syscall.Close(file.fd)
58 file.fd = -1 // so it can't be closed again
59 if e != 0 {
60 return os.Errno(e)
61 }
62 return nil
63 }
<p>
50 func (file *File) Read(b []byte) (ret int, err os.Error) {
51 if file == nil {
52 return -1, os.EINVAL
53 }
54 r, e := syscall.Read(file.fd, b)
55 if e != 0 {
56 err = os.Errno(e)
57 }
58 return int(r), err
59 }
<p>
61 func (file *File) Write(b []byte) (ret int, err os.Error) {
62 if file == nil {
63 return -1, os.EINVAL
64 }
65 r, e := syscall.Write(file.fd, b)
66 if e != 0 {
67 err = os.Errno(e)
65 func (file *File) Read(b []byte) (ret int, err os.Error) {
66 if file == nil {
67 return -1, os.EINVAL
68 }
69 return int(r), err
70 }
<p>
72 func (file *File) String() string {
73 return file.name
69 r, e := syscall.Read(file.fd, b)
70 if e != 0 {
71 err = os.Errno(e)
72 }
73 return int(r), err
74 }
<p>
76 func (file *File) Write(b []byte) (ret int, err os.Error) {
77 if file == nil {
78 return -1, os.EINVAL
79 }
80 r, e := syscall.Write(file.fd, b)
81 if e != 0 {
82 err = os.Errno(e)
83 }
84 return int(r), err
85 }
<p>
87 func (file *File) String() string {
88 return file.name
89 }
</pre>
<p>
There is no implicit <code>this</code> and the receiver variable must be used to access
@ -658,7 +687,7 @@ We can now use our new package:
13 func main() {
14 hello := []byte(&quot;hello, world\n&quot;)
15 file.Stdout.Write(hello)
16 f, err := file.Open(&quot;/does/not/exist&quot;, 0, 0)
16 f, err := file.Open(&quot;/does/not/exist&quot;)
17 if f == nil {
18 fmt.Printf(&quot;can't open file; err=%s\n&quot;, err.String())
19 os.Exit(1)
@ -723,7 +752,7 @@ Building on the <code>file</code> package, here's a simple version of the Unix u
35 cat(file.Stdin)
36 }
37 for i := 0; i &lt; flag.NArg(); i++ {
38 f, err := file.Open(flag.Arg(i), 0, 0)
38 f, err := file.Open(flag.Arg(i))
39 if f == nil {
40 fmt.Fprintf(os.Stderr, &quot;cat: can't open %s: error %s\n&quot;, flag.Arg(i), err)
41 os.Exit(1)

24
doc/go_tutorial.txt
View File

@ -384,8 +384,8 @@ assigned to a variable.
An I/O Package
----
Next we'll look at a simple package for doing file I/O with the usual
sort of open/close/read/write interface. Here's the start of "file.go":
Next we'll look at a simple package for doing file I/O with an
open/close/read/write interface. Here's the start of "file.go":
--PROG progs/file.go /package/ /^}/
@ -437,11 +437,11 @@ We can use the factory to construct some familiar, exported variables of type "*
--PROG progs/file.go /var/ /^.$/
The "newFile" function was not exported because it's internal. The proper,
exported factory to use is "Open":
exported factory to use is "OpenFile" (we'll explain that name in a moment):
--PROG progs/file.go /func.Open/ /^}/
--PROG progs/file.go /func.OpenFile/ /^}/
There are a number of new things in these few lines. First, "Open" returns
There are a number of new things in these few lines. First, "OpenFile" returns
multiple values, a "File" and an error (more about errors in a moment).
We declare the
multi-value return as a parenthesized list of declarations; syntactically
@ -460,6 +460,20 @@ consistent error handling throughout Go code. In "Open" we use a
conversion to translate Unix's integer "errno" value into the integer type
"os.Errno", which implements "os.Error".
Why "OpenFile" and not "Open"? To mimic Go's "os" package, which
our exercise is emulating. The "os" package takes the opportunity
to make the two commonest cases - open for read and create for
write - the simplest, just "Open" and "Create". "OpenFile" is the
general case, analogous to the Unix system call "Open". Here is
the implementation of our "Open" and "Create"; they're trivial
wrappers that eliminate common errors by capturing
the tricky standard arguments to open and, especially, to create a file:
--PROG progs/file.go /^const/ /^}/
--PROG progs/file.go /func.Create/ /^}/
Back to our main story.
Now that we can build "Files", we can write methods for them. To declare
a method of a type, we define a function to have an explicit receiver
of that type, placed

3
doc/progs/cat.go
View File

@ -24,6 +24,7 @@ func cat(f *file.File) {
case nr > 0:
if nw, ew := file.Stdout.Write(buf[0:nr]); nw != nr {
fmt.Fprintf(os.Stderr, "cat: error writing from %s: %s\n", f.String(), ew.String())
os.Exit(1)
}
}
}
@ -35,7 +36,7 @@ func main() {
cat(file.Stdin)
}
for i := 0; i < flag.NArg(); i++ {
f, err := file.Open(flag.Arg(i), 0, 0)
f, err := file.Open(flag.Arg(i))
if f == nil {
fmt.Fprintf(os.Stderr, "cat: can't open %s: error %s\n", flag.Arg(i), err)
os.Exit(1)

3
doc/progs/cat_rot13.go
View File

@ -67,6 +67,7 @@ func cat(r reader) {
nw, ew := file.Stdout.Write(buf[0:nr])
if nw != nr {
fmt.Fprintf(os.Stderr, "cat: error writing from %s: %s\n", r.String(), ew.String())
os.Exit(1)
}
}
}
@ -78,7 +79,7 @@ func main() {
cat(file.Stdin)
}
for i := 0; i < flag.NArg(); i++ {
f, err := file.Open(flag.Arg(i), 0, 0)
f, err := file.Open(flag.Arg(i))
if f == nil {
fmt.Fprintf(os.Stderr, "cat: can't open %s: error %s\n", flag.Arg(i), err)
os.Exit(1)

17
doc/progs/file.go
View File

@ -27,7 +27,7 @@ var (
Stderr = newFile(syscall.Stderr, "/dev/stderr")
)
func Open(name string, mode int, perm uint32) (file *File, err os.Error) {
func OpenFile(name string, mode int, perm uint32) (file *File, err os.Error) {
r, e := syscall.Open(name, mode, perm)
if e != 0 {
err = os.Errno(e)
@ -35,6 +35,21 @@ func Open(name string, mode int, perm uint32) (file *File, err os.Error) {
return newFile(r, name), err
}
const (
O_RDONLY = syscall.O_RDONLY
O_RDWR = syscall.O_RDWR
O_CREATE = syscall.O_CREAT
O_TRUNC = syscall.O_TRUNC
)
func Open(name string) (file *File, err os.Error) {
return OpenFile(name, O_RDONLY, 0)
}
func Create(name string) (file *File, err os.Error) {
return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
}
func (file *File) Close() os.Error {
if file == nil {
return os.EINVAL

2
doc/progs/helloworld3.go
View File

@ -13,7 +13,7 @@ import (
func main() {
hello := []byte("hello, world\n")
file.Stdout.Write(hello)
f, err := file.Open("/does/not/exist", 0, 0)
f, err := file.Open("/does/not/exist")
if f == nil {
fmt.Printf("can't open file; err=%s\n", err.String())
os.Exit(1)